The present invention generally relates to a method for reducing errors in tape drives. More specifically, the invention relates to hard write and read errors in tape drives due to debris on the head or on the tape itself.
Typically, tape drives can be used for storing data from computers, data acquisition systems, and the like. Tape drives are often operated by trained technicians, frequently in controlled environments, to reduce any problems or malfunctions which may arise. However, this is not always the case. Tape drives are now used more frequently being used in a variety of environments by personnel with no special training in tape drive or tape maintenance and care.
Furthermore, tape drives are often required to operate without anyone supervising their operation for an extended period, for example, during back up operations which often take place in the nighttime. Corporate users, like banks, governmental offices, insurance companies and the like often require a tape system which is capable of operating unattended for a period of several hours. This operation is more critical in systems designed as auto loaders, libraries and stackers in which the system automatically picks one tape from a tape magazine without any operator interference.
A major source for failure in such systems is debris which accumulates on the head or on the tape itself. Debris may come from the tape, the tape cassette (cartridge) mechanism, the tape drive or the surrounding air. In addition, human debris is often a source for this problem. Debris which accumulates on the head and/or tape will often lift the tape away from the head. As a result, signal output falls exponentially when the distance between the tape and head increases. Therefore, maintaining both a clean head and a clean tape is important.
This problem is further apparent in high density tape drives which often record at more than 50,000 flux transitions per inch. At these densities, a small amount of debris may significantly reduce the signal amplitude to the extent that the drive cannot read back any data. When performing a read-while-write operation, a tape drive which encounters a hard write error will typically attempt a rewrite operation of the data further down the tape. In many systems, this rewriting operation may be performed as many as sixteen or more times before the system gives up. If the debris is physically on the tape itself, this rewrite procedure will normally work well; however, if the debris is on the recording head, rewriting the data block several times does not typically solve the problem.
During read operations, if the drive encounters a hard read error, it will either try to read the block several times or use error correction characters (ECC) to regenerate the lost data. As long as the debris is on the tape itself, this will normally work; however, with debris on the head, the chances of recovering the data is reduced.
Hard write errors have become a more serious problem than hard read errors. A first reason for this is that tape systems often employ powerful ECC systems which can correct even major read errors. Another reason is that systems often can read signals reliably to less than ten percent of their nominal value. In contrast, when writing, most systems require the minimum signal read back in read-while-write mode to be in the order of thirty-five to forty percent. Therefore, a drive may report a hard write error because the read signal has dropped to, as an example twenty percent, although the drive itself would be able to read such a signal. A tougher requirement on the quality of the read signal during the read-while-write operation insures adequate margins for tapes if they are to be read by other tape drives and/or stored for a long period of time.
Traditionally, a method which is used to remove debris from recording heads includes some form of cleaning, either manually or using some form of cleaning tape or cleaning cassette/cartridge. Such a method is normally effective for removing debris on the head itself, but typically requires an operator to perform at least part of the operation. As previously stated, this is frequently not desirable or not possible. Additionally, if debris is stuck on the tape itself, head cleaning will normally not solve this problem.
To avoid problems with debris on the tape itself, 1/2-inch systems in which tape reels were handled by skilled operators can employ special scrapers built into the tape drive. These scraping devices can be effective in removing debris from the tape. In addition, other systems have been designed with separate scraping units. If a tape system failed with a hard write or read error, the operator may first attempt to clean the head by some manual method. If that did not solve the problem, the operator could then remove the tape reel and run it past a scraping device. These methods, coupled with the fact the systems were operating at fairly low linear densities and with wide tracks, can make such operations reliable.
Today, however, the situation has changed. First, the most popular form of tape drive system is no longer the 178 -inch reel-to-reel system. The 174 -inch tape cartridge is now the most popular form of tape for data storage. Some cartridges can hold more than two GBytes of data. However, the 1/4-inch tape cartridge has the problems related to hard write and read errors. For such high capacity systems, errors are becoming more and more of a problem. This is partially due to the increase in linear density and number of tracks, and further due to the fact that systems are typically operated by non-skilled workers in all kinds of environments. Additionally, since the tape is inside the cartridge, tape cleaning is more difficult. Traditional scrapers cannot normally be used since they may negatively influence the high density recordings.